Conductivity measuring instruments conventionally use a test coil in a bridge circuit coupled to an AC source and to suitable detector means with a control being adjustable to operate an indicator to a null condition, conductivity being indicated on a scale showing the position of the control element. Such instructions require careful and time-consuming adjustments to obtain accurate measurements and rapid accurate measurements have not been possible. In addition, in requiring a null indicator, control element and scale, such instruments are generally quite large and bulky in construction.